The needs to which piston trimmer capacitors are applied have changed substantially. Before the advent of microelectronics these devices were mainly signal, rather than power, elements relied upon for timing. Needed attributes included small size, high precision, and monotonicity. They were infrequently adjusted, so to maintain stability with time their adjusting means were necessarily of stiff action and not of robust construction. Indeed, fragility is so characteristic of the art that a leading providor, the assignee of U.S. Pat. No. 3,584,271, advises electrical connection be made with silver-bearing resin rather than with solder. Those designers intrepid enough to place devices characterized by prior art within a servo- or computer negative feedback loop or other widely varying application have seen them fail rapidly through wear or breakage. Voltage ratings greater than a few hundred are generally not required for signal devices, nor is a great range of adjustment.
Crystals, phase locked loops and digital dividing chains have invaded much of the trimmer capacitor's field of use in small-signal areas, leaving to them power applications such as antenna matching and the like. In these restricted areas of application important parameters shared with signal devices are high quality factor, and monotonicity. Other factors, equally important, not shared with signal devices, are resistance to voltage breakdown, range of adjustment, frequency agility and computer compatibility. High voltage withstanding properties are inherently necessary in power devices. Wide range of adjustment is inherently desirable in an impedance matching device to increase the range of conditions to which it is applied, and frequency agility and the attendant need to adjust the device rapidly and repeatedly is coming forward in the disciplines of radar and nuclear magnetic resonance. Most instrumentation is now based on computer control, so sensitivity to this need dictates trimmer design. Prior art has produced trimming condensers which are designed to be adjusted by the human hand. Computers are capable of making adjustments much more rapidly, so it is reasonable to include in the present specification means to permit a condenser to be very rapidly adjusted without failure.